This invention relates to a dental post construction which can be inserted into a tooth stub and which is utilized to improve retention of a dental restoration built onto the tooth stub.
It is present dental procedure to form a dental prosthetic structure onto a tooth stub for replacement of missing dentition. In this procedure, a tooth stub is initially prepared by removing the diseased or damaged top portion of a tooth to form a tooth stub. A base is formed by drilling into the root canal portion of the tooth stub to form a space into which a dental post can be inserted. Presently available dental post include grooves on their surface designed to improve retention of the post within the tooth stub. Dental cement is employed in the bore in conjunction with the dental post to secure the post in the tooth stub. A portion of the post extends above the tooth stub upper surface so that a dental prosthesis formed on the tooth stub can be retained. Presently, the implantation of a dental post relies either upon the adherent strength of an adhesive or on lateral stress forces between the dental post and the canal wall of screw type posts.
Preformed posts are posts which are premade to specific dimensions with matching burs having cutting surfaces. The burs have a matching diameter to the post and prepare the root to accept a post. A post is then tried in the root and cut to the appropriate length. Cement is spun into the canal with a device referred to as a lenticulo spiral, placed directly with a syringe and/or placed directly on the post. The post is placed in the canal and held in position until excess cement extrudes and the cement hardens. Most preformed posts require placing filling material around the top of the post to transfer strength from the post to the crown. This procedure is referred to as the core build up or post and core procedure.
There are many problems which are encountered when utilizing preformed posts. These include:
An inaccurate fit develops with present bur technology.
Potential for perforation of the root is great with present burs.
There is inadequate resistance to rotational forces on the post.
Root fracture caused by lateral stresses occurs.
There is weak transfer of strength from the post to the crown positioned on the post.
An accurately drilled hole results in good proximity of the post to the canal walls with a thin cement layer to provide greater success in properly positioning the post. The hole is inaccurate if tipping or vibrating of the bur occurs during root preparation as occurs with present drilling systems. This adds to the failure rate of preformed post systems. Drilling a straight hole for a straight post in a curved canal or drilling a hole which does not align with a canal can lead to perforation of a root and loss of a tooth. All posts must resist normal rotational forces which occur during normal or abnormal functions if there is not sufficient tooth structure to provide resistance. In general, preformed posts do not provide good stability against rotational force because they are round and rotate easily when placed in a round hole such as that provided by present bur systems. Presently, to compensate for this, a separate pin may be placed into the tooth, however, screw pins increase the likelihood of root fracture. Some systems try to make posts oval or non-symmetrical at the top but do not supply dependable resistance and retention form. Cement merely provides suction to hold a post in position. The strength of the cement becomes a weak point to the root-post-crown relationship. Constant repeated forces of chewing causes potential breakdown on the tooth-cement-crown interface with subsequent cement wash out and crown post failure. An uneven or excessive amount of force can cause root fracture and tooth loss. Screw type posts can exert large lateral stresses which leads to potential root fracture and tooth loss. In addition, forceful placement of cement type posts without proper venting of cement can cause root fracture and tooth loss. Filling material is placed around a preformed post above the root to accept a crown after the post is cemented. The strength and long term stability of this material becomes a weak link in long term success of the crown. In addition, proper design of the post above the root is critical to resist rotation or dislodging of the filling material from the post.
A cast post is inducted for root canalled teeth with no clinical crown (no tooth above the height of the gums) and/or teeth with root canal spaces which are shaped in such a manner that a preformed post can not fit properly. For example, a canal may be narrow at its bottom half and diverge rapidly in the top half or it may be too oval shaped. The preformed post which is of the same diameter throughout can not accommodate these situations. When utilizing a cast post, root preparation is done by drilling to remove undercuts and obtain slight divergence from the bottom upward. The cast post technique takes an impression of a prepared root canal space. In indirect methods, an impression of the root is taken with a dental impression material. In direct methods, an acrylic pattern of the prepared root and the desired shape above the gums is achieved in the mouth. Laboratory procedures which include casting in a lost wax technique are then necessary to construct the cast post. There are many problems which are encountered when utilizing casts posts. The problems include: An increased chance of root fracture. The cast post is expensive. There is an increased possibility of root perforation. The cast post may not provide good resistance to rotational forces.
All posts need to provide venting of cement as a post is placed. A cast post is very precise fitting so it is difficult for cement to vent, lateral forces can fracture the root and/or the post will not be fully seated as excess cement remains in the bottom. In addition, any bubbles or inaccuracies from the casting process can cause a poor fit and root fracture. Cast posts dramatically increases cost as compared to preformed posts because there are laboratory fees and increased time required to treat the patient. For a cast post, an appointment is needed for an impression in addition to an appointment for post placement. The patient cost of a cast post is double the cost of a preformed post. The doctors laboratory cost may be five to ten times the cost to buy a preformed post.
Preparation of a root canal space must be free of any undercuts or removal of a cast post in its plastic or wax phase of construction will be impossible. It is often difficult to attain this as root canals tend to be complex systems of lateral canals, ribbon shapes, multiple canals, etc. Often, excessive drilling is done which removes important tooth structure and leads to a weaker root and increased chance of root fracture or perforation.
It has been proposed in U.S. Pat. Nos. 4,480,997; 4,490,116 and Re. 31,948 to utilize a threaded dental post which is introduced into the bore of a tooth stub by being rotated to thread the post into position. The dental post includes a stem portion having a slot extending through the stem thickness and along its length which renders the stem being formed of two legs each having its outside surface threaded. The outside surface of the legs intimately contact the walls of the bore so that the threads on the legs can engage the walls. In addition, a spring-like connection for the two legs is provided so that a radial outward spring force is applied to the legs to force them against the bore walls. These dental posts are undesirable since a rotational force must be applied to the post to position it properly into the bore. This positioning process is undesirable since it is time consuming and causes the patient discomfort. In addition, the possibility exists that the post will be threaded too far into the tooth stub which will result in fracture of the tooth stub. Furthermore, the radially outward forces of the legs on the tooth stub can result in fracture of the tooth stub over time.
U.S. Pat. No. 1,534,409 discloses a two legged post having corrugated surfaces which fit into a root canal having generally parallel walls. This surface design materially reduces the post surface area which contacts the canal walls and thus post retention relies primarily upon cement adhesive strength.
Accordingly, it would be desirable to provide a dental post which can be inserted into the bore of a tooth stub while eliminating the need for sole reliance upon lateral stress forces with the canal wall or upon the adhesive strength of an adhesive. In addition, it would be desirable to provide a dental post which distributes force on the tooth so that it is not concentrated on the post or on a pin used in conjunction with the post. Furthermore, it would be desirable to provide a system for utilizing such a dental post which facilitates the placement of a core and a crown.